High frequency electron discharge device and focusing means therefor



June 6, 1967 MCCUNE L 3,324,337

HIGH FREQUENCY ELEGTRON DISCHARGE DEVICE AND FOOUSING MEANS THEREFORFiled D60. 2, 1953 iNV-ENTORS EARL W. MC CUNE LOUIS T. ZITELLI UnitedStates Patent 3,324,337 HIGH FREQUENCY ELECTRON DISCHARGE DE- VICE ANDFOCUSIN G MEANS THEREFDR Earl William McCune, Santa Clara, and Louis T.Zitelli, Palo Alto, Calif, assignors to Varian Associates, Palo Alto,Calif, a corporation of California Filed Dec. 2, 1963, Ser. No. 327,3694 Claims. (Cl. 3l53.5)

ABSTRACT OF THE DISCLOSURE It is possible to obtain uniform flux densityin solenoidal focused high frequency electron discharge devices such asthe klystron and traveling wave tube both along the interaction lengthand within areas wherein wave transmission means such as waveguides arecoupled to the interaction circuit and necessitate an interruption inthe solenoid by incorporating an auxiliary solenoid on the other side ofthe transmission means and providing a re-entrant and surrounding lowreluctance magnetic circuit means for the solenoids.

This invention relates generally to high frequency tubes and moreparticularly'to improvement in klystrons, traveling wave tubes and othersimilar tube structures operable at relatively high frequencies for thegeneration or amplification of radio frequency energy via the mechanismof electromagnetic interaction between an electron beam andappropriately established radio frequency fields.

Extremely serious problems are encountered when attempts are made todesign high frequency tubes of the designated type for operation at highpower levels. As the output radio frequency power level increases, acorresponding increase in the beam power of the tube is requisite. Inturn, the high powered electron beam is intercepted by variousstructural components of the tube wherefore substantial amounts of heatare generated, particularly under conditions of C.W. operation asopposed to intermittent or pulsed operation. By way of example, amulti-cavity klys-tron amplifier arranged for C.W. operation with aradio frequency power output of 100 kw. will require an electron beampower in the neighborhood of 300 kw. Such beam, after traversing theklystron cavities, is conventionally collected by a bucket-likecollector structure in which a considerable amount of heat will begenerated. One problem then is to provide adequate and efficient coolingof the collector.

In addition, heat will be similarly generated in the cavity formingstructures themselves to an extent determined by the amount of beaminterception with such cavity structures. Normally, a magnetic solenoidor the like can be utilized to focus the beam to a pencil-like shapewhich traverses most of the cavities without appreciable interception.However, since the radio frequency power in the final or output cavityof the klystron is normally delivered therefrom through a laterallyprojecting. output waveguide, constituting the preferable structure forhandling radio frequency power at high levels, a physical requirementfor termination of the solenoid at the waveguide exists and the beam, inturn, will spread not only as a result of existent space charge forcesin the beam itself but also as a result of certain transverse forcesexerted on the electrons of the beam by the existent radio frequencyfields in the final or output klystron cavity. As a result, substantialheat is commonly generated in the output cavity of high power klystrons,the amount being sufiicient to change the dimensions of the metallicstructureforming the cavity to an extent such that detuning of thecavity and reduction in overall efficiency of the amplifier results.Similar problems are encountered in traveling wave tubes wherein spacecharge forces can and often do cause serious electron beam impingementproblems, especially in the vicinity of RR coupling sections such astransversely oriented waveguides.

Accordingly, it is a general object of the present invention to providea high frequency tube which incorporates means arranged to control thegeneration of heat by a high power electron beam so that the tube iscapable of operation at very high power levels.

It is a feature of the invention to provide a high frequency tubeincluding novel focusing or confining means for the electron beam sothat the amount of beam interception with the radio frequency fieldsupporting structure is reduced to substantially a nullity.

Yet more specifically, it is a feature of the invention to provide sucha beam focusing structure including several magnetic elements arrangedto maintain a constant magnetic field throughout the interaction regionof the high frequency tube.

Other features and advantages of the present invention will becomeapparent on a perusal of the following specification when taken inconnection with the accompanying drawings wherein:

FIG. 1 is a longitudinal view showing a multi-cavity klystron amplifierembodying the present invention;

FIG. 2 is an enlarged, longitudinal, fragmentary sectional view alongline 22 of FIG. 1, illustrating details of interior construction;

FIG. 3 is a transverse sectional view taken along line 3-3 of FIG. 2;

FIG. 4 is an enlarged fragmentary transverse, sectional view taken alongline 4-4 of FIG. 2; and

FIG. 5 is a longitudinal view showing a traveling wave tube embodyingthe present invention.

With initial reference to FIG. 1, the klystron amplifier generallyincludes an elongated tubular metallic envelope arranged to maintainvacuum conditions therewithin. At one end, an electron gun assembly 12is mounted and arranged to generate and direct a beam of electronslengthwise through the entire tubular structure to a collector assembly14 at its remote extremity. In its transit, the beam traversessequentially four tunable cavity resonators, two of which are indicatedat 16, 22, each arranged with re-entrant drift tube portions definingcavity gaps 16a, 22a, which support standing wave radio frequency fieldsin interacting relationship with the beam.

The radio frequency energy to be amplified is delivered to the firstcavity resonator 16 through a coaxial line 24 and an input coupling loop26, and after amplification the output radio frequency energy istransmitted from the final output cavity 22 through a conventionalwaveguide 28 which is connected to the final cavity resonator through asuitable coupling iris 30 and projects laterally therefrom. Briefly, thewell known amplification mechanism includes an initial velocitymodulation of the electron beam by the input radio frequency energy byway of coupling at the first cavity gap 16a. As the beam travels, abunching or current density modulation results, the effect being furtherenhanced as the electrons traverse the second and third cavity gaps. Thetightly bunched beam of electrons subsequently excites radio frequencyfields in the final or output cavity resonator 22 which energy is thendelivered through the output waveguide 28 to a suitable load (notshown). Further details of the described structure and its operation arenot presented since they are not germane to the present invention andalternate known electron gun and/or interaction structures can besubstituted therefor.

The electron beam during its transit has a tendency to spread both as aresult of space charge forces existent within the beam itself and alsoas a result of force components established within the resonatorcavities, such forces attaining considerable magnitude in the final or 3output cavity resonator 22 where a high radio frequency power level hasbeen developed through the described amplification mechanism. In orderto maintain the beam in a confined pencil-like trajectory during itstransit of the cavity resonators, a beam focusing solenoid 32 ofgenerally tubular configuration surrounds the interaction region of thetube, terminating at one end adjacent the electron gun assembly 12 andat the other end adjacent the laterally projecting output waveguide 28.If the focusing effect of such single solenoid is considered by itself,substantially a constant magnetic focusing field can be establishedthrough the first three cavity resonators of the klystron amplifier butsince the solenoid 32 terminates adjacent the output waveguide 28,fringing fields would exist in the final or fourth cavity resonator 22and the focusing effect will thus diminish considerably and allowsubstantial interception of the electrons with the cavity formingstructure.

In accordance with the present invention, an auxiliary coil 34, as mostclearly illustrated in FIG. 2, encompasses the entrance end of theaforementioned collector assembly 14 so as to be positioned on thelongitudinally opposite side of the output waveguide 28 from thedescribed solenoid 32. More particularly, this auxiliary coil 34surrounds a pole piece 34a integrated with the magnetic circuit 65a,64a, 64b of the main solenoid 32 and functions conjointly therewith sothat ultimately a constant magnetic field exists across the entiremagnetic gap, which with the addition of the auxiliary coil 34,encompasses all of the cavity resonators. Thus, defocusing of theelectron beam in the four cavity resonator 22 is avoided.

Both the solenoid 32 and the auxiliary coil 34 are designed andconstructed in accordance with known techniques wherefore details ofsuch construction need not be spelled out. However, in such design andconstruction, it is preferred that the auxiliary coil 34 be designedwith a number of turns such that the requisite magnetic field isobtained with a current flow identical to that in the main solenoid 32wherefore a single magnet power supply (not shown) can be used and theelectrical circuit can constitute a series connection. Additionally,both the main solenoid 32 and the auxiliary coil 34 can be supplied withcoolant from an identical source (not shown). Thus, the necessity foradditional complexity in both the coolant supply and the power supplyfor the magnetic focusing structures is obviated. Flux return members64a and 64b preferably of a high permeability material such as coldrolled steel, iron, etc., extend between and are integrated with annularpole pieces 65a and 34a as shown in FIGS. 1 and 2 and transverselydisplaced from the longitudinal axis of the tube, along which theelectron beam propagates. Thus the flux from the main solenoid 32premeates the pole piece 34a and together with the flux produced by theauxiliary coil 34 results in a constant flux existing across the entiremagnetic gap between the spaced pole pieces 34a and 65a, without anytransverse magnetic fringe fields existing in the vicinity of thejuncture between the envelope 22' and the output waveguide 28 whichwould, in the absence of the auxiliary coil 34 and the integratedmagnetic circuit 34a, 64a, 64b, 65a, cause expansion of the electronbeam and consequent detuning of the tube. Flux return members 64a and64b serve as low reluctance or magnetically shorted paths for the fluxproduced by the coils. This of course results in an integrated magneticcircuit and reduced leakage and also provides a certain degree ofprotection from extraneous magnetic fields. FIG. shows a traveling wavetube 91 having a conventional cathode 91a, slow wave circuit 91b andcollector 910 members incorporating novel auxiliary coils 92 and 93 andmain coil 94, in conjunction with an integrated magnetic circuitincluding pole pieces 95, 96 and flux return member 97. A matching fluxreturn member is disposed diametrically opposite member 97 in the samemanner as shown for the klystron embodiments of FIGS. 1 and 2. The slowwave circuit 91b is shown as a helix,

but obviously could be any known circuit such as, for example, a discloaded guide, ring and bar, etc. The operation of traveling wave tubesis well known and reference to any suitable text will provide anadequate description of same. Transmission lines 98 and 99 in the formof waveguides provide suitable R.F. coupling means for either forward orbackward amplifier operation. In the case of BWO operation, waveguide 99may be eliminated. As in the case of the klystron embodiment of FIGS. 1and 2, the flux across the gaps between pole pieces and 96 is constantand substantially devoid of transverse components in the vicinity of thetransmission lines 98, 99, which would otherwise result in beamdefocusing and electron impingement on the slow Wave circuit and/o1surrounding vacuum envelope 100.

The described beam focusing arrangement provides for a high percentageof beam transmission to the collector assembly 14 which is arranged toprovide for highly eflicient dissipation of the heat generated duringbeam collection. As best shown in FIGS. 2 and 3, the collector assembly14 is mounted in axial alignment with the remainder of the klystronamplifier and includes a collector bucket 40 composed of a hollowcylindrical tube 42 closed at its remote base end by an internallytapered cup 44, both elements being formed from a good thermalconducting material, such as copper. Additionally, the collector bucket40 preferably includes, adjacent its open entrance end, an inwardlydirected annular flange 46 which serves to minimize re-entry ofsecondary emission electrons from the collector bucket back into theinteraction region of the tube.

The exterior surface of the hollow cylindrical collector tube 42 ismilled to form a plurality of longitudinally extending,circumferentially spaced slots which are covered by a cylindrical copperpartition 48 which is mounted in tightly pressed engagement over theslotted collector tube to thus form coolant channels 50. At its one end,the cylindrical partition 48 is secured to the perimetral edge of anannular flange 52 spaced from the base of the collector bucket 40 asformed by the described cup 44, the inner edge of the flange 52, inturn, being secured to the extremity of a smaller cylindrical tube 54that is axially aligned with the collector bucket 40 and is arranged todeliver coolant to the channels 50, as will be explained in detailhereinafter.

At its other extremity, the cylindrical copper partition 48 terminates ashort distance from an outwardly projecting portion 46a of thepreviously described annular flange 46 at the entrance end of thecollector bucket 40. In a manner somewhat similar to that of thecylindrical collector tube 42 itself, the cylindrical partition 48 ismilled on its exterior surface to provide a plurality of longitudinallyextending and circumferentially spaced slots and is tightly encompassedby a copper sleeve 55 which is secured at its opposite extremitiesrespectively to the exterior edge of the annular flange 46a at theentrance end of the collector bucket 40 and to another annular flange 56that is slightly spaced from the partition supporting flange 52, thusforming return coolant channels 58. The inner edge of this sleevesupporting flange 56 is internally secured to a tube 60 concentricallymounted about the described inlet tube 54 for coolant so as to providean annular outlet passage for the coolant received from the returnchannels 58. As illustrated by the arrows in FIG. 2, coolant, preferablyin the form of distilled water supplied from the inner tube 54, firstmoves radially outward adjacent the base of the collector bucket 40,thence longitudinally through the channels 50 exterior to the collectortube 42 and thereafter through the return channels 58 and into theannular outlet passage in tube 60.

It is to be expressly observed that the coolant flowing through thechannels 50, 58 in both directions is encompassed by copper elements allof which are in direct metallic contact with the collector tube 42itself, as best shown in FIG. 4. Since it is well established that thegreatest impediment to efficient heat transfer exists at an interfacebetween liquid and solid surfaces, the described arrangement whereinonly one such interface exists between all coolant channels and themetal '(i.e., the collector tube 42) from, which heat is to beextracted, the efficiency of heat transfer is optimized.

At its entrance end, the collector assembly 14 is held in spaced,thermally and electrically isolated relation from the remainder of thetube by a ceramic ring 62 positioned between the collector flange 46 anda collector mounting flange 64 disposed adjacent the final cavityresonator 22. At its remote end, the described collector bucket 40 andcoolant structure is provided with an axially flexible connection,generally indicated at 66, and which incorporates another ceramic ring68, for mounting to an encompassing cylindrical shield 70 whichsurrounds the entire collector structure and is brazed or otherwisesecured to the exterior of the pole piece 340 at the terminal end of theinteraction region of the tube.

The flexible connection 66 allows axial motion which results because ofthe difference in thermal expansion between the copper collector and thesteel shell 70. The flexi'ble connection does not allow radial motionand therefore keeps the collector located concentrically within thesteel .shell 70. The shell 70 is made of steel to provide a strongvacuum envelope and also to shield the collector region from undesirablemagnetic fields. X-nadiation is effectively stopped by the coppercollector. Any small amount of radiation emanating from the coppercollector would be further attenuated by the steel shell.

To facilitate ingress and egress of coolant to and from the collectorassembly 14, the mentioned inlet tube 54 projects beyond theencompassing outlet tube 60 and both tubes are provided with circularopenings 54a, 60a in their side walls adjacent their extremities whichare closed respectively by a circular cap 72 and an annular cap 74. Thestepped projecting tubular structure defined by the tubes 54, 60constitutes the male member arranged for pressed connection into thefemale stepped receptacle formed in .a dual coolant manifold 76. Suchmanifold 76 constitutes a genenally cylindrical structure having asmaller bore 78 extending susbtantially half-way therethrough andarranged to encompass the projecting smaller inlet tube 54 and a largerbore 80' extending through the other half thereof and appropriatelydimensioned to encompass the terminal portion of the largerforeshortened outlet tube 60. Each of the bores 78, 80 are centrallyenlarged at a position such that the respective openings 54a, 60a in thewall of the inlet and outlet tubes are encompassed to establish liquidcommunication through inlet and outlet stubs 82, 84 which are arrangedfor appropriate connection to the coolant supply lines (not shown). Toavoid leakage, circular O-rings 79, '81 are supported in sealingrelationship between the bores 78, 80 and the encompassed tubes 54, 60on opposite sides of both the inlet and outlet openings 54a, 60a.

As can be readily visualized, the dual manifold 76 as illustrated, canbe simply pressed over the projecting tubes 54, 60 to establishconnection to the coolant supply. Furthermore, the entire manifold 76can be rotated about the axis defined by the tubes 54, 60 to facilitatepositional arrangement of the coolant connections. Once assembled, themanifold 76 can be held in position on the tubes by a washer 86 that isheld in engagement 'with the manifoldby a thumb bolt 88 arranged forthreaded connection into the end cap 72 on the central tube 54. In viewof the fact that all coolant pressures existent within the manifold 76and the associated inlet and outlet tubes 54, 60 for the coolant areexerted radially, substantially no pressure tending to unseat themanifold 76 from its support on the tubes will exist.

The aforementioned collector assembly is claimed in copending US. patentapplication, Ser. No. 307,989, now Patent No. 3,305,742 entitled HighFrequency Electron 6 Discharge device and Cooling Means Therefor by EarlW. McCune filed Sept. 10, 1963.

By the way of example, a klystron amplifier substantially as illustratedand having an overall length of substantially three feet has beensuccessfully operated at a frequency of approximately two gigacycleswith a C.W. radio frequency output power of 100 kw. and a beam power of300 kw. The described collector assembly 14 has effectively handled theheat generated under such continuous wave operation with a coolant flowof ap proximately 40 gal/min. The described focusing arrangement,supplying a magnetic field of approximately 2000 gauss, hassubstantially eliminated beam interception within the cavities, suchresult being clearly demonstrated by the fact that no observabledifferences in cavity dimensions between operation at kw. and kw. werefound.

While the focusing and beam collection arrangement have beenspecifically described relative to a klystron amplifier wherein standingwaves exist within resonant cavities, it will be apparent that the sameheating problems would be encountered in traveling wave structures, suchas for example, a disc loaded waveguide involving traveling waveinteraction at high power levels.

Since many changes can be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

This application is a continuation-in-part of US. patent applicationSer. No. 308,880, now abandoned by Earl W. McCune and Louis T. Zitelli,filed Sept. 13, 1963 and assigned to the same assignee as the presentinvention.

What is claimed is: p

1. A high frequency tube which comprises an elongated, evacuated tubularenvelope, an electron gun arranged to produce and direct a beam ofelectrons longitudinally therethrough, an interaction structure arrangedto support high frequency fields in interacting relation with saidelectron beam, means for transmitting high frequency energy from saidenvelope adjacent one end of said interaction structure, and at leasttwo beam focusing structures interconnected through a low reluctancepath and surrounding said envelope and positioned on longitudinallyopposite sides of said energy transmitting means, said focusingstructures being arranged to conjointly produce substantially constantfocusing forces on the beam of electrons within the entire interactionstructure, said focusing structures together with said low reluctancepath being arranged to produce magnetic fields which are characterizedby an absence of transverse components of flux in the vicinity of thejuncture between said envelope and said energy transmitting means, saidone of said at least two beam focusing structures being a main solenoidmeans disposed internally of the low reluctance path and extending alongsubstantially the entire interaction structure, said other of said atleast two beam focusing structures being an auxiliary solenoid meansdisposed internally of the low reluctance path, said low reluctance pathhaving a re-entrant portion which extends within the interior of saidauxiliary solenoid, said main solenoid means contributing to themagnetic field in the interior portion'of the main solenoid and in theregion of the juncture between said envelope and said energytransmitting means.

2. A high frequency tube according to claim 1 wherein said main andauxiliary solenoids are electrically arranged for energization by thesame current.

3. A high frequency tube which comprises an elongated, evacuated tubularenvelope, means forming a plurality of longitudinally spaced cavityresonators in said envelope with aligned field supporting gaps, anelectron gun arranged to produce and direct a beam of electronssuccessively through said cavity resonators in interacting relation withfields established in said gaps, means for supplying high frequency waveenergy to the first of said cavity resonators to effect velocitymodulation of the electron beam, a laterally projecting output waveguidecoupled to the last of said cavity resonators to conduct high frequencywave energy therefrom, a beam focusing solenoid encompassing saidenvelope and terminating adjacent one side of said output waveguide, andauxiliary beam focusing means encompassing said envelope on the remoteside of said output waveguide and functioning conjointly with saidsolenoid and interconnected with said solenoid through a low reluctancepath to maintain a substantially constant focusing force on said beamthroughout all of said cavity resonators said beam focusing solenoid andsaid auxiliary beam focusing means together with said low reluctancepath being arranged to produce magnetic fields which are characterizedby an absence of transverse components of flux in the vicinity of thejuncture between the last of said cavity resonators and said laterallyprojecting output waveguide, said beam focusing solenoid and saidauxiliary beam focusing solenoid being disposed internally of said lowreluctance path, said low reluctance path having a re-entrant portionwhich extends within the interior of said auxiliary coil, said beamfocusing solenoid contributing to the magnetic field in the interiorportion of the beam focusing solenoid and in the vicinity of thejuncture between the last of said cavity resonators and said laterallyprojecting output waveguide.

4. High frequency traveling wave tube which comprises an elongated,evacuated tubular envelope, an electron gun arranged to produce anddirect a beam of electrons longitudinally therethrough, an interactionstructure arranged to support high frequency fields in interactingrelation with said electron beam, means for transmitting high frequencyenergy from said envelope adjacent one end of said interactionstructure, and at least two beam focusing structures surrounding saidenvelope and positioned on longitudinally opposite sides of said energytransmitting means, said focusing structures being interconnectedthrough a low reluctance path and arranged to conjointly producesubstantially constant focusing forces on the beam of electrons withinthe entire interaction structure said focusing structures together withsaid low reluctance path being arranged to produce magnetic fields whichare characterized by an absenceof transverse components of flux in thevicinity of the juncture between said envelope and said energytransmitting means, said one of said at least two beam focusingstructures being a main solenoid means disposed internally of the lowreluctance path and extending along substantially the entire interactionstructure, said other of said at least two beam focusing structuresbeing an auxiliary solenoid means disposed internally of the lowreluctance path, said low reluctance path having a re-entrant portionwhich extends within the interior of said auxiliary solenoid, said mainsolenoid means contributing to the magnetic field in the interiorportion of the main solenoid and in the region of the juncture betweensaid envelope and said energy transmitting means.

References Cited UNITED STATES PATENTS 2,619,611 11/1952 Norton et al.315-5.35 2,687,490 8/1954 Rich et al. 315-5.34 2,918,593 12/1959 Rogers31384 2,945,154 7/1960 Bittman et al. 313-84 X 2,974,246 3/ 1961 Beck etal. 3 13-84 3,076,116 1/1968 Drieschman et al. 315-535 HERMAN KARLSAALBAOH, Primary Examiner.

S. CHATMON, JR., Assistant Examiner.

1. A HIGH FREQUENCY TUBE WHICH COMPRISES AN ELONGATED, EVACUATED TUBULARENVELOPE, AN ELECTRON GUN ARRANGED TO PRODUCE AND DIRECT A BEAM OFELECTRONS LONGITUDINALLY THERETHROUGH, AN INTERACTION STRUCTURE ARRANGEDTO SUPPORT HIGH FREQUENCY FIELDS IN INTERACTING RELATION WITH SAIDELECTRON BEAM, MEANS FOR TRANSMITTING HIGH FREQUENCY ENERGY FROM SAIDENVELOPE ADJACENT ONE END OF SAID INTERACTION STRUCTURE, AND AT LEASTTWO BEAM FOCUSING STRUCTURES INTERCONNECTED THROUGH A LOW RELUCTANCEPATH AND SURROUNDING SAID ENVELOPE AND POSITIONED ON LONGITUDINALLYOPPOSITE SIDES OF SAID ENERGY TRANSMITTING MEANS, SAID FOCUSINGSTRUCTURES BEING ARRANGED TO CONJOINTLY PRODUCE SUBSTANTIALLY CONSTANTFOCUSING FORCES ON THE BEAM OF ELECTRONS WITHIN THE ENTIRE INTERACTIONSTRUCTURE, SAID FOCUSING STRUCTURES TOGETHER WITH SAID LOW RELUCTANCEPATH BEING ARRANGED TO PRODUCE MAGNETIC FIELDS WHICH ARE CHARACTERIZEDBY AN ABSENCE OF TRANSVERSE COMPONENTS OF LUX IN THE VICINITY OF THEJUNCTURE BETWEEN SAID ENVELOP AND SAID ENERGY TRANSMITTING MEANS, SAIDONE OF SAID AT LEAST TWO BEAM FOCUSING STRUCTURES BEING A MAIN SOLENOIDMEANS DISPOSED INTERNALLY OF THE LOW RELUCTANCE PATH AND EXTENDING ALONGSUBSTANTIALLY THE ENTIRE INTERACTION STRUCTURE, SAID OTHER OF SAID ATLEAST TWO BEAM FOCUSING STRUCTURES BEING AN AUXILIARY SOLENOID MEANSDISPOSED INTERNALLY OF THE LOW RELUCTANCE PATH, SAID LOW RELUCTANCE PATHHAVING A RE-ENTRANT PORTION WHICH EXTENDS WITHIN THE INTERIOR OF SAIDAUXILIARY SOLENOID, SAID MAIN SOLENOID MEANS CONTRIBUTING TO THEMAGNETIC FIELD IN THE INTERIOR PORTION OF THE MAIN SOLENOID AND IN THEREGION OF THE JUNCTURE BETWEEN SAID ENVELOPE AND SAID ENERGYTRANSMITTING MEANS.